1. Field of the Invention
The present invention relates to stable and high-solids aqueous dispersions of blocked polyisocyanates, either containing auxiliary solvent or being virtually free from auxiliary solvent, to a process for preparing such dispersions and the use of such dispersions as cross-linkers for film-forming resins.
2. Description of the Background
Blocked (or capped) polyisocyanates are known and widely used as cross-linking agents for film-forming resins in solvent-borne coating systems. For reasons of environmental protection and, of course, of cost as well, numerous technical developments of the recent past have been directed to reducing the amount of solvent or, as is particularly desirable, at providing solvent-free or virtually solvent-free systems.
Aqueous dispersions of blocked polyisocyanates are suitable for use in single-component waterborne (or water-thinnable) coating materials, i.e. in aqueous, solvent-free or virtually solvent-free coating systems, which, in addition to the blocked polyisocyanates, which function as cross-linking agents, comprise a water-dissolved or -dispersed film-forming resin having functional groups that are able to react with isocyanate groups. Since blocked isocyanates are, in the majority of cases, solids or highly viscous substances, they can be dispersed in water only with the addition of solvents or, without solvent, only above their melting point. The disadvantages of the two processes are that, in the first case, the solvent used for dilution must be removed again after dispersion, at considerable expense, while in the case of melt emulsification, some of the blocking agent is cleaved off early at the high temperatures required, and the isocyanate groups that have been released react with water in the course of dispersion and are no longer available for crosslinking with the functional groups of the film-forming resin.
The aqueous dispersions of the blocked polyisocyanates (and, of course, those of the film-forming resins as well) should, where possible, be of high solids content. The principal advantage of high-solids content dispersions is that, with only one application, it is possible to produce thick coats having excellent properties (fullness, resistance, hiding power). Relative to multicoat applications, this means not only a saving in time and (because of the multiple stoving procedures) in energy, but also in optimizing quality, since each additional coat increases the risk of production errors. In addition, high-solids dispersions also lessen the costs of transportation and storage. The dispersions should, moreover, be stable, i.e. storable for long periods without sedimenting or coagulating to any marked extent. Should coagulation or sedimentation nevertheless occur, the deposits should be readily and durably redispersible.
Known processes for preparing stable aqueous dispersions of blocked polyisocyanates make use of external emulsifiers and protective colloids (JP 4 216 815, JP 4 216 816, JP 4 216 817 and WO 94/22935). A disadvantage of these processes is the permanent hydrophilicity of the emulsifiers and of the protective colloids, which is retained in the coatings which result on stoving by reaction of the polyisocyanates with the isocyanate-reactive groups of the film-forming resins. Consequently, these coatings have a tendency, under the effect of water, to become cloudy, to soften and/or to swell up, ruling out the use of such dispersions at least for coating materials that are to be employed outdoors.
The process that DE 24 56 469 describes for preparing aqueous dispersions starts with partially blocked polyisocyanates, which are reacted with a hydrophilicizing agent, which contains an NCO-reactive group and also a hydrophilic or potentially hydrophilic group, for example, a sulfo acid or carboxyl group. Both types of groups can be converted by neutralization into hydrophilic ionic groups, for example sulfonate or carboxylate groups. The hydrophilicizing agents used in the examples are the sodium salts of N-methylaminoethanesulfonic acid. The reaction product is then dispersed in water. A similar process is described in EP 0 012 348, where the blocked hydrophilicized polyisocyanates are not only self-dispersible, but also promote the dispersion of hydrophobic film-forming resins. In the case of these proceses, however, which are not used industrially, the linking of the compound having a (potentially) hydrophilic group takes place, because of the partial blocking beforehand, at low NCO contents, which firstly requires an uneconomically long reaction time and secondly has the consequence either that not all of the free NCO groups are hydrophilically linked or, in the case of excess hydrophilicizing agent, that unreacted hydrophilicizing agent remains in the polylsocyanate dispersion.
DE 27 08 611 describes a process for preparing polyurethane prepolymers, in which polyisocyanates are first of all reacted with an excess of hydroxycarboxylic acids such as dimethylolpropionic acid, and then the free isocyanate groups that remain are blocked. These polyurethane prepolymers must have high acid numbers of &gt;30 mg of KOH/g, since at lower acid numbers, stable dispersions or solutions can be prepared only with additional organic solvents. A disadvantage of this process is that a very considerable proportion of the original isocyanate groups is lost for cross-linking with the film-forming resin as a result of the reaction with the hydroxycarboxylic acid hydrophilicizing reagent. A further disadvantage of this process is the high viscosity of dispersions with a solids content &gt;35 % by weight, which makes processing more difficult if not virtually impossible.
EP 0 312 836 describes a process in which isocyanate groups are capped by reaction with a cyanourea salt. The hydrophilic groups thereby created participate in the cross-linking reaction in the course of stoving, and thus do not impair the density of cross-linking. A disadvantage, however, is that amines are given off during stoving, which only with difficulty leave the paint film, and, in the case of light-colored coatings, cause disruptive yellowing of the coatings.
EP 0 133 223 describes blocked polyisocyanates which are hydrophilicized by means of cyclic malonates. Curing in these systems takes place likewise through the cyclic malonate groups, with the result that the hydrophilic modification again does not cause any reduction in the cross-linking density. Detrimental to this process, however, are the considerable amounts of water-soluble organic solvent required.
EP 0 524 511 discloses a process in which nonionically modified polyisocyanates having free isocyanate groups are first of all dispersed in water and then the isocyanate groups are blocked. In the course of this reaction, however, some of the isocyanate groups react with water and are lost for the cross-linking reaction. Moreover, it is difficult to establish reproducibly the particular stoichiometric proportions that are desired.
Furthermore, EP 0 537 578 discloses a process in which blocked polyisocyanates, for textile finishing (crease-resistant finishing), are hydrophilicized with, inter alia, polyalkylene oxide units incorporated therein and are dispersed. EP 0 424 697 and DE 28 14 815 also describe dispersions of blocked, nonionically hydrophilicized polyisocyanates. These dispersions, however, are in turn permanently hydrophilic, so that coatings produced with them may turn cloudy, soften and/or swell under the action of moisture and are, therefore, unsuitable for exterior applications.
EP 0 022 452 describes a one-component polyurethane system whose hardener is a polyurethane prepolymer in which each molecule is linked with a hydrophilic modifying reagent. In addition to the loss of cross-linkable isocyanate groups, a disadvantage here is that a solvent mixture of water and alcohol is required and, in particular, that dispersions with a solids content of &gt;40% by weight have such a high viscosity that they are difficult to process.
Finally, EP 0 566 953 and EP 0 576 952 describe specific polylsocyanate mixtures, hydrophilicized by means of hydroxycarboxylic acids, which are suitable for the curing of film-forming resins in aqueous systems. A need, therefore, continues to exist for an aqueous dispersed polyurethane system of improved stability and high solids content.